Disc brake

ABSTRACT

A disc brake having a caliper, a piston, an operating shaft, a wear adjustment mechanism, and a transmission path. The caliper may straddle a rotor and may retain first and second brake pads. The piston may be slidable within the caliper. The operating shaft may apply an actuating force to advance the piston when the operating shaft is actuated. The transmission path of the wear adjustment mechanism may include a one-way clutch.

TECHNICAL FIELD

The present invention relates to a disc brake. More particularly, butnot exclusively, the present invention relates to an air actuated discbrake including a wear adjustment mechanism.

BACKGROUND

Braking systems of heavy trucks generally use air under pressure as theoperating medium. Typically, large air actuators have a mechanicaloutput which applies brake pads to the brake rotor via a levermechanism. The lever mechanism includes an operating shaft pivotableabout two bearing surfaces on a common axis to urge one or more pistonsagainst a brake pad via a cam surface. The shaft has a lever arm actedupon by the air actuator, and may include another generally shorter armto actuate a wear adjuster mechanism. For larger brake pads, twinpistons are provided to optimize pressure distribution on the brake padbackplate. A known braking mechanism of this type is shown in documentWO2004/074705 (Knorr-Bremse).

For lighter duty applications (e.g., lighter trucks or trailers oftractor-trailer units) with smaller brake pads a single piston issufficient, but nevertheless two bearings are provided. This arrangementadds to the cost and weight of the brake, complexity of machining thebearing surfaces, and also results in problems in packaging thecomponents within the smaller brake envelope, because space at theinboard (with respect to the vehicle on which the brake is fitted) sideof the brake rotor is limited. An example of a brake of this type isEP0730107 (Perrot Bremsen).

The present applicant in EP1852627 B1 proposed a solution to thisproblem. However, the actuation mechanism disclosed therein retains arelatively complex wear adjuster mechanism, cover plate, carrier and padconstruction.

It is therefore desirable to provide an air actuated disc brake having awear adjuster that is simpler, requires fewer parts and is more compact.

The present invention seeks to overcome or at least mitigate theproblems associated with the prior art.

SUMMARY

A first aspect of the present invention provides a disc brake. The discbrake may have a caliper configured to straddle a rotor and retain firstand second brake pads in at least an inboard-outboard direction; apiston slidable within the caliper, for applying a force to the firstbrake pad to move it towards the rotor during a brake operation, thepiston comprising an inner portion restrained from rotating and arotatable outer portion, the inner and outer portions being in threadedengagement, such that rotation of the outer portion of the piston causesthe inner portion of the piston to move axially relative to the outerportion of the piston; an operating shaft for applying an actuatingforce to advance the piston when the operating shaft is actuated; a wearadjustment mechanism for adjusting the clearance between the inboardbrake pad and the rotor, to account for wear of the brake pads androtor; and a transmission path of the wear adjustment mechanism, fromthe operating shaft to the outer piston, wherein the transmission pathcomprises a one-way clutch that directly engages the outer portion ofthe piston, such that when adjustment of the clearance is required,applying the operating shaft causes the one-way clutch to engage theouter portion of the piston and cause the outer portion of the piston torotate, and the inner portion to advance in the direction of the rotor.

As the one-way clutch directly engages the outer portion of the piston,the number of components required is minimized, as there is no need forseparate barrels and gears etc., as required by the prior art.

The one-way clutch may be a wrap spring that encircles the outer portionof the piston. In other embodiments the one way clutch may be of asuitable alternative type, such as ball and ramp/roller clutch/spragclutch arrangements.

Encircling the piston with a wrap spring helps to ensure that thesurface area for frictional engagement is relatively large. This canassist in minimizing wear between the components, increasing the life ofthe brake.

The operating shaft may be supported on a bearing surface of the caliperarranged in line with the piston, such that the actuating force istransmitted in line from the operating shaft to the piston. This mayminimize the machining operations that are needed, and reduce the sizeof the operating shaft.

The disc brake may further comprise a cylindrical roller to transmit theactuating force from the operating shaft to the piston.

The disc brake may further comprise a follower to receive the roller,wherein the follower and the piston comprise opposing faces, the facesarranged to be in slidable contact. This means that no transverse motionof the piston is needed in addition to the axial motion.

The transmission path may comprise a predetermined amount of backlashbetween the operating shaft and the one-way clutch, such that theone-way clutch is not driven until a predetermined running clearance isexceeded.

The transmission path may comprise a friction clutch located between thedrive ring and the one-way clutch, wherein the friction clutch isconfigured to slip if a more than a predetermined force transmittedtherethrough.

The friction clutch may comprise one or more input plates and one ormore output plates arranged in a face-contacting relationship.

The transmission path may comprise a drive ring located between theoperating shaft and the friction clutch, the drive ring may be arrangedto be rotationally driven by the operating shaft about a common axiswith the piston.

At least a proportion of the backlash may be located between the drivering and the friction clutch. Alternatively or in addition, at least aproportion of the backlash may be located between the operating shaftand the drive ring.

The operating shaft may comprise one or more projecting operating arms,and the drive ring may have a forked portion projecting inboard, to beengaged by the one or more operating arms, wherein at least a proportionof the backlash is between the operating arm of the operating shaft andthe forked portion of the drive ring.

The transmission path may comprise a driven ring located between thefriction clutch and the one-way clutch, preferably wherein the drivenring is positioned concentrically between the drive ring and the piston.

The disc brake may further comprise a resilient element for applying aload to the friction clutch.

The disc brake may further comprise a return spring for returning theoperating shaft to its rest position when the operating shaft is notbeing actuated.

The return spring may be concentric with the piston.

The return spring may further act as the resilient element, therebyfurther reducing the part count.

The inner portion of the piston may be coupled with the first brake padto restrain the inner portion from rotation.

The one-way clutch may be a wrap spring, and a first end of the wrapspring may encircle the outer portion of the piston and a second end ofthe wrap spring may encircle an outer surface of the driven ring andengage the outer surface of the driven ring, such that rotation of thedriven ring in one direction is transmitted to the outer piston via thewrap spring.

Encircling the wrap spring around an outer surface of the driven ringhelps to minimize any potential backlash that may occur between thedriven ring and the wrap spring, which can help to reduce wear of thecomponents.

A second aspect of the present invention provides a rewind mechanism fora brake adjuster, the mechanism comprising a rewind shaft arranged todrivingly engage an adjuster mechanism and a head accessible on a brakecaliper for engagement by a user to permit the shaft to be turned eitherdirectly or by use of a tool, the shaft being supported for rotation ona bearing support, the bearing support being formed from sheet metalmaterial, the sheet metal material having a cylindrical section toengage a face of a bore of the caliper and a radial portion comprising abore to receive the shaft.

The second aspect may optionally have sealing material on an outer faceof the cylinder, preferably overmolded thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is an isometric view of a brake according to an embodiment of thepresent invention;

FIG. 2 is an isometric side cross-sectional view of the brake of FIG. 1on a radial inboard-outboard plane;

FIG. 2A is a close-up view of a portion of the brake of FIG. 2;

FIG. 3 is an isometric plan cross-sectional view of the brake of FIG. 1;

FIG. 4 is a side cross-sectional view through the brake of FIG. 1 on aradial inboard-outboard plane offset from a center line of the brake;

FIG. 5 is an isometric view of the actuator arrangement of the brake ofFIG. 1;

FIG. 6 is an exploded view of the brake of FIG. 1;

FIG. 7 is an isometric view of the carrier and brake rotor of the brakeof FIG. 1;

FIG. 8 is an isometric view of the caliper of the brake of FIG. 1;

FIG. 9 is a cross-sectioned plan view of the caliper of the brake ofFIG. 1;

FIGS. 10 and 11 are exploded perspective views of outboard and inboardbrake pads and of the brake of FIG. 1;

FIG. 12 is a detail view of a mouth of a cylindrical bore of the caliperwith an alternative arrangement for mounting a guide insert;

FIG. 13 is an isometric view of the brake according to FIG. 1, but withan alternative embodiment of spreader plate;

FIG. 14 is a detail isometric view of the brake pads and spreader plate,with the brake pads in an unworn condition;

FIG. 15 is the same view as FIG. 14, but with the brake pads in a partworn condition;

FIG. 16 is a similar view to FIG. 14, but with a pad retainer clip shownin place;

FIG. 17 is an axial view looking inboard of the inboard brake pad,spreader plate, pad retainer clip and pad strap of FIG. 16;

FIG. 18 is an exploded view of an alternative wear adjuster mechanismarrangement;

FIG. 19 is a close up of the exploded view of FIG. 18;

FIG. 20 is an isometric view of the assembled wear adjuster mechanism ofFIG. 18; and

FIG. 21 is a side cross-sectional view through a portion of an assembledbrake, which includes the wear adjuster mechanism of FIG. 18, on aradial inboard-outboard plane through a center line of the brake.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

Actuation

FIGS. 1, 2 and 3 illustrate a disc brake 2 of the present invention. Thedisc brake incorporates an actuation mechanism comprising a singlepiston suitable for a commercial vehicle, for example a trailer of atractor-trailer combination.

Various orientations of the disc brake are described. In particular thedirections inboard and outboard refer to the typical orientation of thedisc brake when fitted to a vehicle. In this orientation the brake padclosest to the center of the vehicle is the pad directly actuated by anactuation mechanism and being the inboard pad, and the outboard padbeing one mounted to a bridge portion of the caliper. Thus inboard canbe equated with an actuating side of the disc brake, and outboard with areaction side. The terms radial, circumferential, tangential and chordaldescribe orientations with respect to the brake rotor. The termsvertical and horizontal describe orientations with the disc brakemounted uppermost on an axle, whereas it will be appreciated that in usesuch a disc brake may adopt any axle orientation depending uponpackaging requirements of the vehicle.

The disc brake 2 comprises a caliper 3 having a housing 6 to accommodatethe actuation mechanism and which is slideably mounted on a carrier 4for movement in an inboard-outboard direction.

An inboard brake pad 11 a comprises a layer of friction material 13facing the rotor 10, is mounted to a spreader plate 12, and is describedin more detail below. The pad 11 a is moveable in the direction of arrow14 against the brake rotor 10.

Suitable means are provided to urge an outboard brake pad 11 b againstthe opposite side of the rotor 10. In this embodiment, such meanscomprises a bridge 5 arranged so as to straddle a rotor 10 and totransmit the reaction force from an inboard operating shaft 21 to theoutboard pad 11 b. The bridge 5 further defines an opening through whichthe brake pads 11 a, 11 b can be fitted and removed radially. In thisembodiment the housing 6 and bridge 5 are manufactured as a singlemonolithic casting, but in other embodiments, the bridge may be boltedor otherwise secured to the housing.

With reference to FIGS. 2, 2A and 3 in particular, the inboard actuationmechanism comprises a single brake piston 15 slideable in the directionof arrow 14 (i.e., inboard-outboard) relative to the rotor 10.

In order to urge the piston assembly in the direction of arrow 14, theoperating shaft 21 is pivoted about a transverse axis 22 within a saddleon a single bearing surface 27 of the caliper. The operating shaft 21supports a roller 23 whose axis 28 is parallel and offset from the axis22 and couples with a follower 20 in a semi-circular recess thereof. Asurface 19 of the follower 20 opposing the recess is in slideablecontact with an inboard end face of the piston 15 so that no transversemovement of the piston is required.

The operating shaft 21 further comprises a lever 24 having a pocket 25adapted to receive an output push rod (not shown) of a brake actuator(e.g., an air chamber). The lever 24 is, in this embodiment, shaped asan inverted “U” (see FIG. 5 in particular) and the line of action of thebrake actuator (from pocket 25) is substantially over the line of actionof the piston 15.

In other embodiments, another form of cam surface instead of roller 23may be employed (e.g., a plain bearing) and/or the arrangement may bereversed with the follower 20 or roller 23 being in contact with thecaliper housing 6.

Application of a force in the direction of arrow 26 (FIG. 5) causesarcuate movement of the operating shaft 21 and thus the roller 23 bearson the follower 20 and the follower in turn bears on the piston 15 tourge the brake pad 11 a directly against the rotor 10. A reaction forcefrom the operating shaft 21 is transmitted to the bearing surface 27 ofthe caliper 3 via a needle roller bearing (not shown) and is thentransmitted to an outboard pad 11 b via the bridge 5, with the outboardpad being urged against the rotor 10, such that the pads 11 a and 11 bclamp the rotor and effect braking through a frictional drag force.

Wear Adjustment

A wear adjuster mechanism 30 to maintain a desired running clearancebetween the rotor 10 and pad 11 a, 11 b is described below. Withparticular reference to FIGS. 2A and 6, the operating shaft 21 includesan operating arm 33 which extends from the side thereof parallel to theaxis of rotation. The operating arm 33 is in driving engagement with afork portion 34 of a drive ring 35 that is arranged concentrically withthe piston 15.

In this embodiment, the drive ring further comprises two opposingrecesses 37 which face inboard and are arranged to accommodate acorresponding pair of lugs provided on input plates of a friction clutch39. In other embodiments alternative arrangements for driving the clutchinput plates are contemplated e.g., a different number of recesses,projections rather than recesses, or arm 33 being extended to directlydrive the input plates.

The friction clutch further comprises output plates positioned betweenthe input plates. The output plates of the clutch 39 have a number ofradially inwardly facing lugs 40 (not present on the input plates) thatengage in corresponding recesses 41 in a driven ring 42 that in thisembodiment is nested concentrically between the drive ring and thepiston 15.

The piston 15 comprises an outer piston 15 a having a female thread,with an inner piston 15 b having a complimentary male thread locatedwithin it.

On the outboard facing edge of the driven ring 42, a relatively smallnotch 43 is provided that connects to one free end of a wrap spring 44.The wrap spring 44 is arranged to frictionally wrap around an outerportion 15 a of the piston 15 and acts as a one-way clutch. In otherembodiments other suitable one-way clutches may be utilized, such asball and ramp, roller clutch/sprag clutch arrangements.

The components above between the operating arm and inner and outerpiston define a transmission path of the wear adjuster mechanism.

The wear adjuster mechanism 30 additionally comprises a washer 45retained by a circlip 46 held in an annular recess of the outer piston15. An outboard face of the washer 45 is acted on by a return spring 47,the return spring being arranged to retract the piston 15, and thereforethe brake pad when a brake actuation cycle ceases. The inboard face ofthe washer 45 supports a compression spring 48 arranged concentricallybetween the drive ring 35 and driven ring 42 in order to load the clutch39 and generate the required amount of friction that controls, thetorque at which it slips.

The inboard end of the outer piston portion 15 a further has a gearwheel 49 mounted thereto for use with a rewind mechanism, described inmore detail below.

The inner piston portion 15 b is provided with a pin 50 (FIG. 3) to fitwithin a complimentary slot 55 in the spreader plate 12 and therebyprevent rotation of the inner piston in use.

In order to maintain a desired running clearance between the brake padsand rotor the wear adjuster mechanism is required to periodicallyadvance the inboard brake pad 11 a towards the rotor 10 to account forthe loss of friction material 13 due to wear.

In a normal braking operation in which the running clearance is withinthe desired parameters, as the operating shaft 21 pivots, the operatingarm 33 causes the drive ring to rotate. However, a predetermined amountof play or backlash is provided in the system between the recesses 37and corresponding lugs 38 on the clutch, and as such the drive plates ofthe clutch do not rotate and no adjustment will occur.

If the running clearance is however greater than the desired range, theaforesaid play is taken up and rotation of the drive plates on thefriction clutch 39 will occur. Whilst there is excess running clearanceto be taken up, this rotation is transmitted via the driven plates anddriven ring to the wrap spring 44, causing the wrap spring to rotatearound the outer piston portion 15 a in a direction which causes thewrap spring to tighten, transmitting the rotation to the outer piston.

Since the inner piston portion 15 b is restrained from rotation, thiscauses a lengthening of the piston to reduce the running clearance. Atthe point at which the friction material 13 comes into full contact withthe rotor 10 further rotation of the outer piston portion 15 a isprevented and the friction clutch 39 slips. Once the braking operationceases and the return spring 47 acts to push the operating shaft 21 backto its rest position, a corresponding retraction of the inner pistonportion 15 b is prevented since the wrap spring 44 relaxes and does nottransmit a reverse rotation to the outer piston portion 15 a.

The co-axial mounting of the adjuster mechanism 30 around the piston 15minimizes the space required by the mechanism within the housing,resulting in a lighter, more compact housing. The use of the outersurface of the piston as a barrel for the wrap spring, obviates the needfor the separate barrels and gears of the prior art and minimizes thenumber of components required for the brake to function. The outerpiston 15 a is required to be manufactured from a strong and durablematerial (e.g., high tensile steel) to close tolerances in order totransmit the clamp load to the brake pad, and is therefore also suitablefor use as such a barrel without significant adaptation, save foradditional hardening thereof in most circumstances.

An alternative wear adjuster mechanism 130 is described below. Likefeatures are indicated in the figures with like reference numerals, butwith the prefix ‘1’. With particular reference to FIGS. 18 to 21, theoperating shaft 121 includes an operating arm 133 which extends from theside thereof parallel to the axis of rotation. The operating arm 133 isin driving engagement with a fork portion 134 of a drive ring 135 thatis arranged concentrically with the piston 115.

In this embodiment, the drive ring further comprises an axiallyextending outer lip having two opposing recesses 137 which face outboardand are arranged to accommodate a corresponding pair of radially outwardextending lugs 138 provided on input plates 139 a of a friction clutch139. A further recess can also be seen in FIG. 18, but in thisembodiment has no operational function; it simply assists withmanufacture of the drive ring, allowing it to be easily pressed from aflat plate to create the axially extending outer lip. In otherembodiments, alternative arrangements for driving the clutch inputplates 139 a are contemplated e.g., a different number of recesses,projections rather than recesses, or arm 133 being extended to directlydrive the input plates.

The friction clutch 139 further comprises output plates 139 b positionedaxially between the input plates. The output plates 139 b of the clutch139 have a number of radially inwardly facing lugs 140 (not present onthe input plates) that engage in corresponding recesses 141 in a drivenring 142 that in this embodiment is nested concentrically between thefriction clutch 139 and the piston 115.

In this embodiment, the friction clutch 139 is made up of two inputplates 139 a, and two output plates 139 b located between the inputplates.

The piston 115 comprises a hollow outer piston 115 a having a femalethread located on its inner surface, with an inner piston 115 b having acomplimentary male thread on an outer surface thereof.

A plurality of projections 141 a extends radially from an outer surfaceof the driven ring 142. The recesses 141 are defined between theseprojections 141 a. The projections only extend axially along the drivenring 142 for approximately half its axial length, i.e., an outboard halfof the outer surface is smooth. This outboard outer surface of thedriven ring 142 provides a surface for locating a first longitudinal endof a wrap spring 144, which frictionally wraps around the outboard outersurface of the driven ring 142. The second longitudinal end of the wrapspring 144 is arranged to frictionally wrap around an outer portion 115a of the piston 115 and act as a one-way clutch.

In this embodiment, the outer portion 115 a of the piston includes ashoulder portion 116 projecting radially from a central part of theouter portion 115 a of the piston 115. Specifically, it is this shoulderportion 116 that the wrap spring 144 engages and is frictionally wrappedaround. As can be seen most clearly from the close up cross-sectionalview of FIG. 21, the first end of the wrap spring 144 is wrapped aroundthe smooth surface of the driven ring 142, and the second end of thewrap spring 144 is wrapped around the shoulder portion 116 of the outerportion 115 a of the piston 115. An outboard end surface of the drivenring 142 abuts the shoulder portion 116 when the brake is assembled.

The components above, between the operating arm and inner and outerpiston, define a transmission path of the wear adjuster mechanism.

In this embodiment, the wear adjuster mechanism also includes thrustbearings 163. A first thrust bearing 163 engages an outboard surface ofthe friction clutch 139, and a second thrust bearing 163 engages aninboard surface of the drive ring 135. The thrust bearings 163 areannular, and include circumferentially distributed spherical rollingelements 163′, which assist with free rotation of the clutch mechanismand drive ring 135, to help reduce friction within the system and wearof the components.

The wear adjuster mechanism 130 additionally comprises a washer 145 thatengages the first thrust bearing 163 that is located adjacent the clutch139. An outboard face of the washer 145 is acted on by a return spring147, the return spring 147 being arranged to retract the piston 115, andtherefore the brake pad when a brake actuation cycle ceases. The forceapplied by the return spring 147 to the inboard face of the washer 145also acts to load the clutch 139 (via the thrust bearing 163) andgenerate the required amount of friction that controls the torque atwhich it slips.

The inboard end of the outer piston portion 115 a further has a gearwheel 149 mounted thereto for use with a rewind mechanism, described inmore detail below.

The operation of the wear adjustment mechanism is as described above(previously with reference to FIGS. 2 and 6A), except the driven ring isno longer in communication with the wrap spring via a tang of the wrapspring engaging with a notch of the driven ring; the wrap springdirectly engages the outer surface of the driven ring.

This helps to minimize any potential backlash that may occur between thedriven ring and the wrap spring, which can help to reduce wear of thecomponents. Uncontrolled unwinding of the spring is minimized, whichprovides a more predictable, and hence controllable, friction level. Thewear on the outer surfaces of the piston and driven ring is alsoreduced. This could, for example, obviate the need to heat treat theseouter surfaces, reducing manufacturing costs. The arrangement also helpsto remove any uncertainties that may arise after manufacturing of thecomponents, for example unpredictable tolerances, which may affect thefunction of the system.

The co-axial mounting of the adjuster mechanism 130 around the piston115 minimizes the space required by the mechanism within the housing,resulting in a lighter, more compact housing. The use of the outersurface of the piston 115 a as a barrel for the wrap spring, obviatesthe need for the separate barrels and gears of the prior art andminimizes the number of components required for the brake to function.The shoulder portion 116 of the piston 115 a retains the driven ring 142in the axially outboard direction within the mechanism, and theprojections 141 a on the driven ring 142 limit movement of the wrapspring 144 in the axially inboard direction, without the need forseparate circlips, or other similar retaining components.

Rewind Mechanism

Once the friction material 13 has worn to its design limit, it isnecessary for the brake pads 11 a and 11 b to be replaced. In order toaccommodate the extra depths of unworn new pads as compared to worn oldpads, it is necessary for the piston 15 to be rewound back to itsretracted position.

To this end, a rewind shaft 51 is provided, having a bevel gear 52mounted at an inner end thereof to mesh with the gear wheel 49. A hexhead 53 or other suitable interface is provided at the outer (useraccessible) end to which a spanner, wrench or other suitable tool may beattached to effect the rewinding operation.

In one embodiment, the shaft 51 is mounted within a transversecylindrical opening 57 cast or machined in the caliper. A support 54including a plain bearing rotatably supports the shaft. In thisembodiment the support 54 is formed in a sheet metal e.g., sheet steelhaving a cylindrical portion, a truncated cone portion and a disc shapedportion all formed monolithically from a single piece of metal in adrawing or pressing operation.

A peripheral seal (not shown) may be bonded to the outer face of thecylindrical section without the use of adhesive, due to it being formedas an overmolding, causing it to be secured thereto by solidification ofthe sealing material when liquid during the molding operation. In otherembodiments a separate sealing component, such as an O-ring, may beused. Additionally, in some embodiments the shaft may have multiplesupports along its length.

In this embodiment, a helical spring 55 may be provided between thesupport 54 and hex head of 53 of the rewind shaft 51 so as to bias thebevel gear 52 out of engagement with the gear wheel 49, to reduce therisk of accidental rewinding of the wear adjuster mechanism 30.

It will be appreciated by those skilled in the art that the mountingarrangement for the rewind shaft 51 maintains effective sealing of thecaliper whilst reducing the amount and mass of the parts required.Further the opening 57 may additionally be utilized to provide accessfor machining of the bearing surface 27.

Piston Guidance

Referring now in particular to FIGS. 2, 2A, 3, 8 and 9, the mountingarrangement of the piston 15 and adjustment mechanism 30 within thecaliper 3 is apparent. It can be seen from FIG. 3 in particular that thecavity within which the actuation mechanism is mounted is essentially“T” shaped in plan view, comprising a first cylindrical bore 56extending inboard-outboard, and the second partially cylindrical bore 57extending transverse to the first and intercepting therewith.

In certain embodiments the cavity may be formed by casting usingsuitable cores and little or no machining, in other embodiments thecavity may be formed solely by machining from the rotor side inboard toform cylindrical opening 56 and transversely to form the opening 57. Inother embodiments a combination of casting and machining may be used. Inall of these embodiments, it will be apparent however that the cavity56, 57 is a simple shape compared with those of the prior art resultingin simplified manufacturing of the caliper 3.

Additionally, since the bore 56 is cylindrical, the complex cast andmachined cover plates of the prior art are no longer required to closeoff the rotor side of the cavity and prevent the ingress of foreignmatter.

In place of cover plates, a pre-machined cylindrical guide insert 60 isutilized, which is provided with a machined bore 61 to guide the piston15.

The guide insert 60 further comprises an annulus 62 closed at anoutboard end to act as a pocket for receiving and guiding the returnspring 47. With reference to FIG. 12, a radially outward facing recessis also provided proximate the outboard end thereof in which a sealingelement such as a convoluted sealing boot 64 may be secured.

The guide insert 60 may further comprise a circumferential recess 67 inthe outer circumferential surface thereof to accommodate a circlip/snapring 68, as well as a further annular recess to accommodate an O-ringtype seal 59 in order to prevent foreign matter from entering the joinbetween the guide insert 60 and caliper. The bore 56 is, in thisembodiment, provided with a circumferential groove 59 at an appropriatelocation into which the circlip/snap ring can relax and hold the guideinsert 60 into place.

Various arrangements are contemplated for mounting the guide insert 60into the caliper 3. These arrangements include mounting the guide insert60 via a taper fit, interference fit or threaded fit to the caliper 3,or by using an additional fastener such as the aforementionedcirclip/snap ring or a canted clip, or a radially oriented bolt as a setscrew. In one embodiment, as shown in FIGS. 2 and 2A, an intermediateinternally threaded sleeve 58″ is cast into or otherwise secured intothe caliper housing. The guide insert 60″ has a complementary externallythreaded surface which enables it to be screwed into the bore 56. Inthis embodiment the sleeve 58″ is additionally provided with a liphaving a recess to accommodate the sealing bolt 64.

However, with reference to FIG. 12, in a preferred embodiment thecaliper 3 is cast with a metal insert 65 protruding from the facethereof adjacent to where the guide insert 60 is to be fitted.

The metal insert 65 is shaped such that it is keyed into the cast metalonce solidified and is malleable such that it is able to be peened overa lip the outboard face of the guide insert so as to hold the insert 60in place. In preferred embodiments, the metal insert 65 extends aroundthe entire periphery of the bore 56 (i.e., it is substantiallycircular). In FIG. 12, the upper section of the metal insert 65 is notyet peened, whereas the lower portion is shown peened over a lip 66 onthe insert. The peening process may itself generate a sufficient sealthat no additional sealing, such as the O-ring 59, is required.

In a preferred embodiment a secondary means of securing the guide insert60 is preferably provided. In order to remove and replace the guideinsert 60, for example during a maintenance or a remanufacturingoperation, the peened material needs to be permanently removed.Consequently it cannot be used for retaining a replacement guide insert60. Therefore, as mentioned above, the circlip/snap ring 68 may beutilized as such a secondary retaining arrangement when a replacementguide insert is fitted.

Thus, whilst FIG. 12 shows both the peened metal insert 65 and the snapring 68 in use, in practice, for a first installation the peened insertonly would be used, and for a replacement the snap ring 68 only would beused. As such an original fit guide insert may lack the groove 67whereas an aftermarket replacement may lack the lip 66.

It will be appreciated that part or all of the actuation and adjustmentmechanism may be advantageously be fitted to the caliper 3 as apre-assembled unit, together with the guide insert 60 via thecylindrical opening 56. This assembly method is possible due to thecompact nature of the actuation mechanism, and in particular theoperating shaft with a single central bearing surface facing the inboardface of the housing 6.

One advantage of the peening approach or circlip/snap ring mounting isthat assembly of the insert, together with the piston 15, operatingshaft 21 and wear adjuster mechanism 30 is easier to achieve with thefit between the insert and caliper being relatively loose, in order thatthe parts may be aligned in the correct location for subsequentoperation. This may be easier to achieve with these mounting methods,than with alternative threaded or interference fits.

Brake Carrier

Referring now to FIGS. 1, 3, 5, 6 and 7, the carrier 4 can be seen inmore detail.

A significant difference between the carrier 4 of the present inventionand those of the prior art is that the carrier is formed from twoportions 4 a and 4 b and a gap or space is present where a connectingbeam conventionally connects a leading side of the inboard brake pad 11a support to a trailing side.

A further significant difference between the present carrier 4 and priorart carriers is that the carrier does not include a support for theoutboard pad. Rather, the outboard pad 11 b is supported directly on thecaliper 3 radially and circumferentially. Additionally, due to theincreased turning moment caused by the outboard pad drag load beingdirected through the caliper 3 rather than the carrier 4, the carrierfurther comprises support wings 70 a and 70 b extending at leastpartially over the rotor 10, and being able to contact and support thecaliper 3 during a braking operation.

Considering the carrier 4 in more detail, in this embodiment, thecarrier portions 4 a and 4 b generally have mirror symmetry about aninboard-outboard plane defined by a central radial axis R thereof.Portion 4 a comprises a generally planar main body 71 a having twothreaded bores 72 extending inboard-outboard. These bores are providedto mount the portion 4 a to a suitable bracket on a rotationally fixedportion of a vehicle such as an axle or steering knuckle (not shown) viabolts (not shown). In other embodiments the mounting may be“tangential”—i.e., the bores extend in generally the same direction inwhich a brake pad is fitted and removed from the carrier. Moreover, thenumber of bores may be altered as required, and/or keying features maybe provided to positively engage the carrier portion with the bracket.

The main body 71 a additionally comprises a threaded bore (not visible)for mounting a guide sleeve 73 a thereto for the caliper 3 to be able toslide thereon in an inboard-outboard direction.

An L-shaped profile is additionally formed in the portion 4 a so as toprovide a vertical (circumferential) abutment surface 75 a to transferthe drag load when the brake is applied from the inboard brake pad 11 aand transmit the load to the axial steering knuckle. The L-shapeadditionally provides a horizontal (radial) abutment surface 76 a so asto support the inboard brake pad 11 a in a radially inward direction,and maintain its correct position with respect to the rotor 10. In orderto avoid a stress raising angle, a curved recess (not visible forportion 71 a but visible in portion 71 b) is provided in thisembodiment. The carrier does not extend circumferentially further thanthe horizontal abutment surface, thus saving weight.

A wing 70 a extends out over the rotor 10 in a direction substantiallyat 90° to the major plane of the main body 71 a. The wing 70 a defines asubstantially vertical guide surface 77 a arranged so as to take andtransfer circumferential loads induced by twisting of the caliper 3under braking to the carrier directly, rather than via the guide sleeves73 a and 73 b. The surface may be machined to achieve a smoothcontrolled surface finish.

It will be appreciated that in prior art carriers, the beaminterconnecting the horizontal abutment surfaces 76 a and 76 b has aprimary purpose of maintaining the spatial relationship between theleading and trailing edge portions of the pad abutments. In thisembodiment, such a spatial relationship may be maintained due to theinterface between the support wings and the caliper for transportationand fitting instead.

The present applicant has recognized that maintaining this spatialrelationship is beneficial for transportation of a carrier and calipercombination, during installation on a vehicle. However a number ofbenefits accrue to the separation of the carrier into the two portions 4a and 4 b. Firstly, this enables the portions to be manufactured as aforging rather than a casting together with the wings 70 a and 70 b,something that would not be readily achievable if a single integral andmonolithic carrier were to be manufactured. For high volumes ofcomponents, forging may be more cost-effective, and the resultantcomponents may be stronger and require less machining than comparablecast components.

Secondly, it will be appreciated that the abutment surfaces 75 a, 76 a,75 b, 76 b typically require some machining and in some circumstancesheat treating. In particular, the machining to achieve smooth planarsurfaces is simplified in the arrangement of the present invention sinceeach portion can be located rigidly during the machining operation,meaning that deflection is reduced, and accuracy of machining istherefore easier to achieve.

Thirdly, the portions 4 a and 4 b require less space in transit when notassembled into the brake, since they can be stacked more efficiently,reducing shipping costs. In order to avoid the assembled caliper andcarrier combination becoming misaligned during shipping, a suitablespacer may be provided that prevents the carrier portions rotating in anunwanted manner about the guide pins.

It would be appreciated that some of the aforementioned benefits willcontinue to be present when the carrier portions are used in conjunctionwith an outboard carrier portion that is bolted to the inboard portion,or otherwise secured thereto, and takes the load from the outboard pad.

It will additionally be noted that the guide pin 73 a is longer than theguide pin 73 b as pin 73 a is the leading pin and intended to take themajority of the load from the caliper. However, the leading and trailingpins can simply be reversed by unbolting them from the position shown inFIG. 7 and switching their positions. In all other respects, portions 4a and 4 b are identical mirror images in this embodiment.

The absence of the beam may also enable, in some embodiments, the brakepads to be restrained from radial outward motion due to a suitable“dovetail” (undercut) formation on the radially inner edge of thefriction material, backplate (if present) and/or a spreader plate suchthat the usual pad strap may not be needed for this purpose. To releasea worn pad, bolts may be removed from one or both of the portions 4 a, 4b, the portion may then pivot (either about a remaining bolt and theguide sleeve, or about solely the guide sleeve if all bolts are removedon one of the portions), to a position where the pad is released and anew one may be fitted. In one embodiment, one carrier portion 4 a may besecured by a single bolt and the other secured by two bolts. Therefore,only a single bolt may need to be removed in order to release the brakepad. This arrangement may be particularly beneficial where the brake ismounted at a 5 o'clock or 7 o'clock position, both of which arecommonplace in trailer applications.

In variants, the first and second portions may differ in the shapeand/or materials used. For example, the portion that supports thetrailing edge of the pad in the usual direction of rotation may bestronger than the portion supporting the leading edge, either due to astronger material being used, the portion being subjected to a differenttreatment, or to the portion having a different shape to impart morestrength.

Further the guide sleeve may be integrally formed with the main body oreach portion as part of one single casting, or the guide sleeve may bepress-fitted or friction welded therein. Due to the simpler overallshape of each portion as compared to a conventional carrier, integralcasting of the guide sleeves may be easier to achieve.

Brake Pads

Turning now to FIGS. 1, 3, 6, 10 and 11, the brake pads of the presentinvention can be seen in more detail. As mentioned above, the outboardbrake pad 11 b is supported entirely by the bridge 5 of the caliper 3.

A support surface 80 is provided in the bridge 5 in a spaced parallelrelationship with the rotor 10, and facing the rotor. In this embodimentthe surface 80 has a key formation for the pad in order to mechanicallyrestrain the pad from motion in a circumferential direction and in aradial direction. In this embodiment the formation comprises a series ofcircular depressions 81 which mate with complimentary circularprojections 82 on a support face 93 of the pad. In other embodimentsother suitable key formations may be utilized.

Conventionally, brake pads for air actuated disc brakes of the type usedin heavy vehicles are typically provided with a metallic backplate thatimparts additional strength, rigidity and toughness to the pad as awhole and is fused and/or mechanically connected to the frictionmaterial. The backplate also typically enables a pad spring to bemounted to the brake pad in order to resiliently restrain the pad fromradially outward motion in use. However, the present applicant hasrecognized that backplates add to the weight, volume and cost of brakepads and in certain situations may not be required.

Therefore, as in the present instance where it is possible to supportthe friction material across its entire surface area on the bridge 5,and where impact loads to abutment surfaces of a carrier will not beexperienced, it is possible to dispense with the backplate.Nevertheless, in order to ensure the outboard pad 11 b is held inposition during operation of the brake, a retaining mechanism to holdthe pad to the surface 80 is required.

To that end, in this embodiment, the outboard pad 11 b is provided withbearing surfaces in the form of tongues 83 extending the entire heightof the vertical end faces of the pad 84. The tongues 83 are providedproximate the outboard face having projections 82 and are relativelythin in proportion to the depth of the pad as a whole. For a typical padhaving a depth of 21 mm, the depth of the tongues is typically around 5mm, but may be in a range of 8 mm to 3 mm, preferably 6 mm to 4 mm.

As can be seen in FIGS. 3 and 9 spaced, parallel chordally extendingbores 85 are formed in the bridge 5 outboard of the support surface 80,aligned to the intended position of tongues 83 when the pad 11 b isinstalled.

To hold the outboard pad 11 b in position a pad retainer clip 86 isprovided which comprises an upwardly arched resilient pad spring portion87, each end of which terminates in a downwardly extending resilientforked retainer section 88 having two prongs 89 biased towards eachother. One prong 89 of the fork engages behind the respective tongues 83of the pad and other fits within the adjacent bore 85 behind supportsurface 80. Thus the pad 11 b is held at each circumferential endagainst the support surface in an outboard direction.

The friction material 13 itself is intended to act as the bearingsurface (tongue) having sufficient mechanical strength to be held to thesupport surface 80. However friction material is a composite materialand typically has a low tensile strength. It is nevertheless desirablefor the bearing surface to be as thin as possible so as to minimize theamount of friction material that cannot be utilized and remains unwornbecause the prongs 89 are in contact with the rotor 10. Therefore, thetongue portions 83 and the surrounding sections of the main body of theoutboard brake pad may be strengthened by utilizing an increasedproportion of steel fibers, or the like within the composite material toimprove its performance in tension. In other embodiments, the bearingsurface may be formed by a bore extending through some or all of theheight of the pad parallel to the bore 85.

In this embodiment, the inboard pad 11 a is of identical constructionand shape to the outboard pad 11 b, also comprising a tongue 83projecting from the vertical end faces 84 thereof. However, withreference to FIG. 11 it will be appreciated that the pad 11 a isintended to be mounted against the spreader plate 12 manufactured fromsubstantially rigid steel or cast iron material that is utilized touniformly transfer the load from the piston to the inboard pad 11 a withlittle or no flexion thereof.

Channels 91 are provided in the rear (inboard) face of the spreaderplate to accommodate and hold one of the prongs 89 of the retainer 86,the other being in engagement with the tongue 83 of the pad 11 a in asimilar manner to the tongue 83 of pad 11 b. A similar key formation ofdepressions 81 in the backplate and projections 82 in the frictionmaterial is utilized to engage the pads to the backplate.

Fitting of the pads 11 a, 11 b to the spreader plate or housing isachieved firstly by removing a pad strap 92 spanning the caliper housingand bridge, sliding the pad in radially inwardly with suitable clearancebetween the rotor and spreader plate 12 or support surface 80, aligningthe projections 82 and depressions 81, and then sliding the pad retainerclip 86 radially inwardly with one prong extending over the tongues 83and the other locating in the channel 91 or bore 85. As such, fittingand removal may be achieved with the caliper and rotor in-situ on avehicle.

In this embodiment, pads 11 a and 11 b being identical reduces thenumber of individual parts needed for the brake, in other embodimentsthis need not be the case. In some embodiments, different materials maybe utilized for the inboard and outboard pads, e.g., to achievedifferent friction coefficients and therefore loading on the carrier andbridge components, and potentially reducing taper wear by reducing themoment generated by the outboard pad. In this instance, the keyformations inboard and outboard may differ to prevent the pads beingfitted in the wrong position.

In other embodiments, the dimensions of the pads 11 a and 11 b maydiffer in addition to, or instead of differences in material. In certainembodiments two or more discrete pads may be mounted to the bridge 5 orthe spreader plate 12 using individual retainer clips or a single commonclip for each side. This may be desirable where one pad has frictionmaterial suitable for use in a particular operating condition (e.g.,high temperature or wet conditions) whereas another may be suited forother conditions. Further, using pads with different wearcharacteristics adjacent to each other may be another means ofcontrolling taper wear. Specifically, taper wear tends to result in thetrailing edge of the outboard pad wearing more than the leading edge,but the leading edge of the inboard pad wearing more. Therefore mountinga pad made from relatively lower wear material in a trailing position atthe outboard location, and in a leading position at the inboard locationmay tend to equalize wear across both inboard and outboard pads. Inother embodiments a single pad formed from different friction materialsarranged side by side may be utilized, optionally with a commonbackplate.

In embodiments where multiple discrete pads are to be mounted,additional clips (optionally without the pad spring portion) may retainthese pads in additional bores or channels provided in the caliper orspreader plate respectively.

In alternative embodiments the retainer clips described above may beutilized with pads that do have a metallic backplate, which backplateforms the tongues. It is however anticipated that the backplate would besignificantly thinner than those of the prior art, leading to many ofthe benefits outlined above.

Alternative forms of pad retainer clip may be used which do not includethe pad spring section, and so are separate for each circumferentialend, or which clip the pad to the support in a different manner, such asbeing clipped together in a circumferential direction rather than aradial direction. In variants only a single retainer clip may berequired, e.g., at one circumferential end of the pad, with the tongueat the other end fitting into a complementary fixed slot.

Spreader Plate Guiding

In a further development of the spreader plate illustrated in FIGS. 13to 17, the spreader plate is configured to be guided on the pad strap92. Like parts are labelled by like numerals in FIGS. 13 to 17 bycomparison with FIGS. 1 to 12. Parts that differ from the embodiments ofFIGS. 1-12 have the suffix ′.

As noted above one potential problem with single piston brakes, ascompared to twin piston brakes, is that of uneven or so-called “taperwear” caused by the greater tendency of the pad to pivot relative to therotor, due to reduced support from the piston head at the rear thereof.

In this embodiment, the spreader plate 12′ is reconfigured to includefirst and second guide bars 95′ that are secured in a radially outwardposition to the main body of the spreader plate 12′ which is intended totransmit load to the friction material. The guide bars 95′ extendoutboard over the inboard pad 11 a, and in this embodiment inboard to alesser extent. Thus, whereas the spreader plate has a depth of around 10mm in this embodiment, the guide bars 95′ have a depth inboard-outboardof between 35 and 40 mm in this embodiment, preferably around 36-38 mm.By being radially outward of the main body the bars 95′ do not contactthe rotor 10 as the friction material 13 is worn, and in addition thebars are radially adjacent to the pad strap 92.

The guide bars 95′ are spaced from each other by slightly more than thewidth of the pad strap 92. The guide bars in this embodiment haveradially extending abutment surfaces 96′ facing inwardly towards the padstrap 92 that are machined to a smooth finish to assist in sliding alongthe corresponding radially orientated surfaces of the pad strap, bothduring braking operations, and as the inboard brake pad 11 a advancesoutboard to account for the wear of the brakes pads 11 a, 11 b and therotor 10, as is illustrated by comparing the positions of the inboardpad in FIG. 15 by comparison with FIG. 14.

Due to the relatively close running fit between the guide bars 95′ andpad strap 92, and the increased depth of the bars compared to thespreader plate 12, it will be appreciated that the degree to which thespreader plate 12, and therefore the inboard brake pad 11 a can pivotabout a radial axis is significantly limited, thereby limiting thedegree to which taper pad wear is able to occur.

FIGS. 16 and 17 illustrate a variant of the pad retainer clip 86′ which,in order to avoid the guide bars 95′ has a pad spring section that isoffset inboard from the forked sections to come into contact with anunderside of the pad strap, as well as being bowed radially outwardly.However, in other embodiments the pad spring portion may no longer beconnected to the forked sections 88′, and may be positioned solelybetween the guide bars 95′ to be resiliently loaded against the padstrap 92.

It will be appreciated that the spreader plate 12′ may be guided on thepad strap 92 by use of other guide formations. For example, the padstrap may be formed with one or more longitudinal slots within it intowhich a suitable portion projecting from the spreader plate may fit, orthe guide bars may be shaped so they at least partially encircle the padstrap, and the spreader plate is lifted out together with the pad strap.In other brakes, the backplate of a pad itself may be provided with asimilar guiding arrangement, instead of a spreader plate.

It will be appreciated that numerous changes may be made within thescope of the present invention. For example, certain aspects of theinvention may be applicable to other types of brake, such as twin pistonor electromechanically actuated brakes (e.g., the brake pads and thecarrier aspects).

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A disc brake comprising: a caliper configured tostraddle a rotor and retain first and second brake pads in at least aninboard-outboard direction; a piston slidable within the caliper forapplying a force to the first brake pad to move the first brake padtowards the rotor during a brake operation, the piston including aninner portion restrained from rotating and a rotatable outer portion,the inner and outer portions being in threaded engagement such thatrotation of the outer portion of the piston causes the inner portion ofthe piston to move axially relative to the outer portion of the piston;an operating shaft for applying an actuating force to advance the pistonwhen the operating shaft is actuated; a wear adjustment mechanism foradjusting a clearance between the first brake pad and the rotor toaccount for wear of the first and second brake pads and the rotor; and atransmission path of the wear adjustment mechanism that extends from theoperating shaft to the outer portion of the piston, wherein thetransmission path comprises a wrap spring that encircles the outerportion of the piston and directly engages an outer surface of the outerportion of the piston that encircles the inner portion of the pistonsuch that when the clearance is adjusted application of the actuationforce by the operating shaft causes the wrap spring to engage the outersurface of the outer portion of the piston, causes the outer portion ofthe piston to rotate, and causes the inner portion to advance in thedirection of the rotor.
 2. The disc brake of claim 1 wherein the innerportion of the piston is coupled with the first brake pad to restrainthe inner portion from rotation.
 3. The disc brake of claim 1 whereinthe operating shaft is supported on a bearing surface of the caliperarranged in line with the piston such that the actuating force istransmitted in line from the operating shaft to the piston.
 4. The discbrake of claim 1 further comprising a cylindrical roller to transmit theactuating force from the operating shaft to the piston.
 5. The discbrake of claim 4 further comprising a follower that receives the roller,wherein the follower and the piston comprise opposing faces arranged tobe in slidable contact.
 6. The disc brake of claim 1 wherein thetransmission path further comprises a predetermined amount of backlashbetween the operating shaft and the wrap spring such that the wrapspring is not driven until a predetermined running clearance isexceeded.
 7. The disc brake of claim 6 wherein the transmission pathfurther comprises a friction clutch located between a drive ring and thewrap spring, wherein the friction clutch is configured to slip if a morethan a predetermined force transmitted therethrough.
 8. The disc brakeof claim 7 wherein the friction clutch further comprises one or moreinput plates and one or more output plates arranged in a face-contactingrelationship.
 9. The disc brake of claim 7 wherein the drive ring islocated between the operating shaft and the friction clutch and thedrive ring is arranged to be rotationally driven by the operating shaftabout a common axis with the piston.
 10. The disc brake of claim 9wherein at least a proportion of the backlash is located between thedrive ring and the friction clutch.
 11. The disc brake of claim 9wherein at least a proportion of the backlash is located between theoperating shaft and the drive ring.
 12. The disc brake of claim 11wherein the operating shaft has one or more projecting operating armsand the drive ring has a forked portion projecting inboard to be engagedby one or more projecting operating arms, wherein the backlash isbetween the one or more projecting operating arms of the operating shaftand the forked portion of the drive ring.
 13. The disc brake of claim 9wherein the transmission path further comprises a driven ring locatedbetween the friction clutch and the wrap spring, wherein the driven ringis positioned concentrically between the drive ring and the piston. 14.The disc brake of claim 13 wherein a first end of the wrap springencircles the outer portion of the piston, and a second end of the wrapspring encircles an outer surface of the driven ring and engages theouter surface of the driven ring such that rotation of the driven ringin one direction is transmitted to the outer portion of the piston viathe wrap spring.
 15. The disc brake of claim 7 further comprising aresilient element for applying a load to the friction clutch.
 16. Thedisc brake of claim 15 further comprising a return spring for returningthe operating shaft to a rest position when the operating shaft is notbeing actuated.
 17. The disc brake of claim 16 wherein the return springis concentric with the piston.
 18. The disc brake of claim 17 whereinthe return spring further acts as the resilient element.
 19. A discbrake comprising: a caliper configured to straddle a rotor and retainfirst and second brake pads in at least an inboard-outboard direction; apiston slidable within the caliper for applying a force to the firstbrake pad to move the first brake pad towards the rotor during a brakeoperation, the piston including an inner portion restrained fromrotating and a rotatable outer portion, the inner and outer portionsbeing in threaded engagement such that rotation of the outer portion ofthe piston causes the inner portion of the piston to move axiallyrelative to the outer portion of the piston; an operating shaft forapplying an actuating force to advance the piston when the operatingshaft is actuated; a wear adjustment mechanism for adjusting a clearancebetween the first brake pad and the rotor to account for wear of thefirst and second brake pads and the rotor; and a transmission path ofthe wear adjustment mechanism that extends from the operating shaft tothe outer portion of the piston, wherein the transmission path comprisesa one-way clutch that directly engages the outer portion of the pistonsuch that when the clearance is adjusted application of the actuationforce by the operating shaft causes the one-way clutch to engage theouter portion of the piston, causes the outer portion of the piston torotate, and causes the inner portion to advance in the direction of therotor; wherein the transmission path further comprises a predeterminedamount of backlash between the operating shaft and the one-way clutch,such that the one-way clutch is not driven until a predetermined runningclearance is exceeded; and wherein the transmission path furthercomprises a friction clutch located between a drive ring and the one-wayclutch, wherein the friction clutch is configured to slip if a more thana predetermined force transmitted therethrough.